Epitaxial growth of AlN by plasma-assisted, gas-source molecular beam epitaxy
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Monocrystalline AlN(OOOl) films with few defects were deposited on vicinal a(6H)-SiC(0001) wafers via plasma-assisted, gas-source molecular beam epitaxy within the temperature range of 1050-1200 °C. The Al was thermally evaporated from an effusion cell. An electron cyclotron resonance plasma source was used to produce activated nitrogen species. Growth on vicinal Si(100) at 900-1050 °C resulted in smooth, highly oriented AlN(0001) films.
I. INTRODUCTION Aluminum nitride possesses a direct band gap of 6.28 eV at 300 K,1 a melting point in excess of 2275 K,2 and a thermal conductivity of 3.2 W/cm • K.3 As such, it is a candidate material for high-power and high-temperature microelectronic and optoelectronic applications with the latter employment being particularly important in the ultraviolet region of the spectrum.1 This material also has the highest reported surface acoustic wave velocity (Raleigh VR = 6-6.2 km/s, VL = 11-12 km/s 4 ~ 6 ) for any material and a substantial electromechanical coupling coefficient (to 1%7). These properties strongly indicate that superior surface acoustic wave devices, operational in aggressive media and under extreme conditions both as sensors for high temperatures and pressures and as acousto-optic devices, can be developed.8"10 However, progress regarding these (and other) applications is hampered by the lack of good single crystal material. The primary objective of the research reported below has been to address this issue via the fabrication of thin films of this material via molecular beam epitaxy (MBE) techniques. In previous studies, mono- and polycrystalline films of AIN have been grown by chemical vapor deposition (CVD) using NH 3 and A1(CH3)3 or A1C13 on a ( 6 H ) - S i C , u sapphire,1'9'12 and Si.13"45 Chu et al.11 obtained smooth monocrystalline AIN layers to a thickness of 25 fim on a(6H)-SiC{0001} substrates by CVD from 1200-1250 °C. A high density of defects in these AIN films was revealed by chemical etching. In general, films grown on sapphire and Si substrates possessed a much rougher morphology than those grown on a(6H)-SiC. This occurred very likely because the difference in lattice
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Present address: Naval Research Laboratory, Code 6861, 4555 Overlook Avenue, SW, Washington, DC 20375-5320.
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http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 9, Sep 1993
Downloaded: 11 Mar 2015
parameters between AIN and SiC is substantially less than between AIN and sapphire or AIN and Si. Gas-source MBE using electron beam evaporated Al and NH3 (Ref. 16) or thermally evaporated Al and plasma-derived activated nitrogen species17 has also been used for single crystal AIN growth. Yoshida et al.16 obtained single crystal AIN using an Al effusion cell and NH3 at 1000-1200 °C on S i ( l l l ) and Al 2 O 3 (0001) and (0112) and obtained growth rates of up to 1 fim/h. They contended that their films were much smoother than CVD-grown material and rivaled bulk single crystal AIN. Sitar et al.17 used an electron cyclotron resonance (ECR) plasma for decomposition of N 2
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